1. Field of the Invention
This invention relates to a charging device for an electro-photographic image forming apparatus, such as a laser printer, a copying machine, a facsimile machine, etc. This invention specifically relates to a contacting type charging device.
2. Description of the Related Art
Conventionally, a corotron charger using a corona discharger and a scorotron charger are generally used to charge an electrophotographic copying machine. FIG. 13 shows a conventional corotron charger 100. The corotron charger 100 includes a wire electrode 101 having a diameter of 100 .mu.m or less. A shield electrode 102 includes an opening for exposing an electrophotographic member (or charge target) 103 to be charged by the corotron 100. The shield electrode 102 surrounds the wire electrode 101 and is grounded. When a voltage of about 6 kV is applied to the wire electrode 101 by a power source 104, corona discharge occurs in the vicinity of the wire electrode 101, which charges the surface of the charge target 103. A scorotron charger (not shown) having a grid electrode has been used to charge a photosensitive member in the electrophotographic copying machine because uniformity of charging directly affects image quality.
However, these types of chargers used in the electrophotographic copying machine generate ozone, which is harmful to the human body and has a bad odor. Therefore, users of these devices desire a new charger, in place of the corotron charger and the scorotron, which does not have these disadvantages. In addition, the corotron charger and the scorotron charger require a high supply voltage of about 6 kV. Thus, a problem often occurs with these chargers in that a large load is applied to the power source.
In order to solve the above problems, a contactable charging device for charging the surface of the charge target has been proposed. This known contacting-type charging device has a conductive member which contacts the surface of the charge target. This charging method requires a low supply voltage and produces only an extremely small amount of ozone. Using this contactable charging device, the supply voltage can be reduced and the occurrence of ozone can be minimized.
One conventional contacting-type charging device has a conductive member for supplying a voltage and which is formed of elastic material such as rubber. This known device contacts the charge target by the elastic force of the rubber. However, if the conductive member is left for a long time in contact with the charge target, the contact force of the conductive member against the charge target becomes weak, because of fatigue of the rubber. In order to solve this problem, Japanese Laid-open Patent Publication No. 2-282280 discloses a method in which a contacting-type charging member is pushed against a charge target using the elastic flexibility of a leaf spring.
FIG. 12 shows such a leaf-spring type contacting-type charging device 90. The leaf-spring type device 90 has a leaf spring 91 which is fixed to a conductive support member 93 by a presser member 94 and a screw 95. The leaf spring 91 is arranged to be biased toward and contacting a photosensitive layer 97 provided on the surface of a photosensitive drum serving as a charge target. The photosensitive drum includes the photosensitive layer 97 coated on the surface of an aluminum tube 98. An elastic resistant layer 92 is formed on the free end of the leaf spring 91 and has a portion which contacts the photosensitive layer 97. Further, a cleaning blade 99 is provided above the periphery of the photosensitive drum to remove dust such as toner, paper powder, etc. The leaf spring 91 is formed of stainless steel having a thickness of about 100 .mu.m. The resistant layer 92 is formed of urethane rubber or nitrile rubber (NBR) and has a thickness of about 50 to 100 .mu.m and a resistance value of about 10.sup.3 .OMEGA.cm to 10.sup.15 .OMEGA.cm. The support member 93 is connected to a negative electrode of a DC power source 96 through an electric wire. In general, a voltage of about -500 V to -2000 V is applied from the DC power source 96 to the resistant layer 92.
In this contacting-type charging device, in order to enable the resistant layer 92 of the leaf spring 91 to uniformly contact the photosensitive layer 97, the leaf spring 91 must be provided with the proper flexibility. However, in the method for supporting one side of the leaf spring 91, as shown in FIG. 12, if the leaf spring 91 does not have sufficient rigidity, so that it is sufficiently biased against the photosensitive layer 92, the resistance layer 92 floats away from the photosensitive layer 97. In this case, it is very difficult to achieve uniform contact between the resistant layer 92 and the photosensitive layer 97. Therefore, the resistant layer 92 must be designed to have sufficient rigidity so that floating is prevented.
However, such a rigid elastic member has large frictional resistance. Thus, it induces a critical problem in abrasion of the resistance layer 92 and the photosensitive layer 97. Therefore, durability of the charging device is poor. On the other hand, if the frictional resistance of the surface of the resistant layer 92 is reduced to prevent abrasion, the corresponding reduction in the rigidity makes it difficult to prevent floating between the resistant layer 92 and the photosensitive layer 97. Further, in the conventional contacting-type charging device as described above, when paper powder, which cannot be removed by the cleaning blade 99, is inserted into a gap between the resistance layer 92 and the photosensitive layer 97, the charge target is not uniformly charged.